This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast which can be dimmed by an external dimmer such as used with incandescent lamps.
A gas discharge lamp, such as a fluorescent lamp, is a non-linear load to a power line, i.e. the current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the current will increase rapidly unless there is a ballast in series with the lamp to limit current.
Because of the non-linear characteristics of a gas discharge lamp, dimming has long been a problem and many solutions have been proposed. Most dimmers include complicated circuitry and all dimmers require external access to the ballast, e.g. by wire connecting the ballast to a dedicated control circuit, a knob on a control shaft extending from the ballast, or optical sensors electrically coupled to the ballast. Until now, gas discharge lamps could not be controlled by dimmers intended for incandescent lamps, e.g. diodes, triacs, or variacs.
The simplest dimmer for an incandescent lamp is a diode in series with the lamp. The diode cuts off the positive portion or the negative portion of the A.C. waveform, thereby reducing the power applied to the lamp. Only two light levels are available with a diode, dim and bright. A triac dimmer uses switching circuitry to cut off an adjustable portion of the A.C. waveform to change the power delivered to a lamp. A variac is a variable transformer which reduces the voltage to a lamp for a range of light levels. A variac differs from a triac in that the output voltage from a variac is sinusoidal. Since many electronic ballasts require a sinusoidal line voltage in order to operate, a variac may seem a likely candidate for dimming a gas discharge lamp driven by an electronic ballast.
A variac is large, heavy, and expensive and not used for dimming lighting in residential or commercial applications. Dimmers must be more compact, lighter, and less expensive than variacs. Typical dimmers use one or more semiconductor switches to block a portion of the line voltage. Dimmers can be divided between those which block the initial portion of the AC cycle and those which block the terminal portion of the AC cycle.
The AC line voltage has a sinusoidal waveform and crosses zero volts twice per cycle. A triac dimmer blocks the line voltage from the zero crossing to some predetermined time after zero crossing, then passes the line voltage. The delay is usually expressed in degrees and, if the delay is 90.degree., a triac is turned on at the peak voltage of the power line, e.g. 170 volts for a 120 volt power line. Many electronic devices, such as ballasts, have capacitive inputs. Switching on at or near the peak line voltage produces a large in-rush of current to such devices, causing a significant and undesirable amount of electrical and acoustical noise.
Dimmers which block the terminal portion of the AC cycle are known as "soft" dimmers, or "quiet" dimmers, or "electronic" dimmers, or "capacitive" dimmers. The latter term shall be used herein. Capacitive dimmers typically include field effect transistors and a zero crossing detector. The transistors are turned on at each zero crossing and turned off at a predetermined point each half cycle to vary the average power supplied to a load. Many commercially available capacitive dimmers are based upon the T5555 zero crossing detector sold by SGS-Thompson Microelectronics.
The simplest ballast for a gas discharge lamp is a resistor in series with the lamp but the resistor consumes power, thereby decreasing efficiency of the lighting system, measured in lumens per watt. A "magnetic" ballast is an inductor in series with the lamp and is more efficient than a resistor but is physically large and heavy. A large inductor is required because impedance is a function of frequency and power lines operate at low frequency (50-60 hz.)
An electronic ballast typically includes a converter for changing the alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 khz. Since a frequency much higher than 50-60 hz. is used, the inductors in an electronic ballast can be much smaller than the inductors for a magnetic ballast.
Converting from alternating current to direct current is usually done with a full wave or bridge rectifier. A filter capacitor on the output of the rectifier stores energy for powering the inverter. The voltage on the capacitor is not constant but has a 120 hz "ripple" that is more or less pronounced depending on the size of the capacitor and the amount of current drawn from the capacitor.
Some ballasts include a boost circuit between the rectifier and the filter capacitor in the converter. As used herein, a "boost" circuit is a circuit which increases the DC voltage, e.g. from approximately 170 volts (assuming a 120 volt line voltage) to 300 volts or more for operating a lamp, and which may provide power factor correction. "Power factor" is a figure of merit indicating whether or not a load in an AC circuit is equivalent to a pure resistance, i.e. indicating whether or not the voltage and current are sinusoidal and in phase. It is preferred that the load be the equivalent of a pure resistance (a power factor equal to one). Electronic ballasts have a significant advantage over magnetic ballasts because a magnetic ballast has a poor power factor.
Most electronic ballasts sold today do not dim properly, if at all, in response to a reduced line voltage. A gas discharge lamp is essentially a constant voltage load on a ballast and, if lamp current decreases, the voltage across the lamp increases slightly. Consequently, most electronic ballasts stop working abruptly when the line voltage is reduced below a certain level. Thus, a variac cannot be used to dim gas discharge lamps driven by most electronic ballasts.
Some regulated electronic ballasts operate a lamp at constant power by drawing more current at reduced line voltages. Electrical utilities often control power distribution on a grid with "brown-outs" in which the line voltage is reduced by up to ten percent in some or all of the grid. Regulated power supplies, including ballasts, not only interfere with a utility's ability to control power consumption but make the problem worse by drawing even more current at reduced voltage in order to maintain constant power to a load; e.g. U.S. Pat. No. 4,220,896 (Paice). Unfortunately, the alternative has been to let gas discharge lamps flicker or go out. It is desired that an electronic ballast dim in response to reduced line voltage, thereby helping utilities to achieve their intended purpose with brown-outs.
There are many types of electronic ballasts and a preferred embodiment of this invention includes what is known as a series resonant, parallel loaded inverter. Such inverters avoid the necessity of an output transformer by coupling a lamp in parallel with the capacitor of a series resonant inductor and capacitor. The inverter typically oscillates at a frequency slightly higher than the resonant frequency of the inductor and capacitor and dimming is achieved by raising the frequency of the inverter. The resonant output provides a sinusoidal voltage for the lamp.
It is a characteristic of series resonant, parallel loaded inverters of the prior art that the frequency of the inverter decreases as the line voltage decreases. For example, U.S. Pat. No. 4,677,345 (Nilssen) describes a series resonant, parallel loaded inverter including a "half bridge, " i.e. series connected switching transistors. A saturable reactor is connected in the base-emitter circuit of each transistor for switching the transistors at a frequency determined by the saturation time of the reactors. If the line voltage decreases, the reactors saturate more slowly and the frequency of the inverter decreases. As the frequency decreases, the series inductor presents less impedance and prevents lamp current from decreasing in proportion to line voltage. Thus, output power is relatively insensitive to line voltage.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast which can be controlled by a capacitive dimmer connected between the ballast and a power line.
A further object of the invention is to provide an electronic ballast having a converter and an inverter which reduces power to a gas discharge lamp in response to reduced voltage from the converter.
Another object of the invention is to provide an electronic power supply including an inverter which provides less power in response to a reduced supply voltage independently of the voltage applied to the power supply.
A further object of the invention is to provide an electronic ballast for gas discharge lamps which can be on the same branch circuit as incandescent lamps and controlled by a single dimmer.
Another object of the invention is to provide an inverter in which the frequency of the output current increases as the voltage supplied to the inverter decreases.
A further object of the invention is to provide a series resonant, parallel loaded inverter in which the frequency of the inverter is approximately inversely proportional to the supply voltage.
Another object of the invention is to provide an electronic ballast which can be dimmed by varying the output voltage from a boost circuit in the ballast.